Mass transfer is the transfer of mass from a relatively high concentration to a relatively low concentration. Typically the term is used to describe physical processes that involve molecular and convective transport of atoms and molecules within physical systems. Mass transfer of fluids, both gases and liquids, is a key requirement in a large number of industries such as chemical, petroleum refining, gas processing, pharmaceutical, and food and includes both separation and mixing. The typical separation techniques include distillation, high pressure absorption, and extractions (including supercritical fluid extractions). For mixing applications there are many processes where gas-gas, gas-liquid and liquid-liquid contacting is important. Gas must be efficiently and effectively contacted with the liquid to provide mass transfer (absorption or desorption; absorption of gas into a liquid to produce a chemical reaction is often a particularly important function). For both separation and mixing a key component for this mass transfer operation is to have an effective mass transfer device. The typical devices include trays, random packing and structured packing.
For random packing, e.g. FIG. 1, the shapes that are typically used include a saddle such as Koch-Glitsch Super INTALOX® Saddle and ring styles such as Sulzer Nutter Ring. Their respective shapes are designed to minimize liquid holdup and pressure drop. The external geometry prevents the column packing from interlocking or entangling, ensuring the randomness and optimum surface area within the packed bed, while the internal fingers, arches and vanes promote optimum interfacial gas/liquid contact with minimal drag or hold-up. Energy consumption is reduced, due to lower reflux ratios. In high pressure operations and in situations where fouling is a concern, the use of random packings is usually preferred. Random packings are also usually preferred for absorption systems.
Modern structured packings, e.g. FIG. 2, take the form of corrugated sheets (mostly metallic) arranged in an orderly manner to allow downward liquid flow and upward gas flow. Advances have been made both in structured packing design with products such as: Koch-Glitsch HC, Montz M and MN, and Sulzer MellapakPlus series. Corrugated structured packings are essentially film flow type devices and they work efficiently as long as a continuous film is held by surface tension. The intersections of the corrugated sheets create mixing points for the liquid and vapor phases. Significant effort has focused on trying to minimize pressure drop and thereby increase capacity while maintaining separation efficiency. Various optimizations, such as changing the corrugation angle and inclination angle, have been based on metal sheets. The basic design of packing requires that the elements be rotated from one element to the next to provide additional mixing but this rotation from one element to the next results in additional pressure drop.
There are limits to the current separation structures of trays, corrugated sheets, saddles and rings. These structures all require flow transition from one packing element to the next, e.g. from one tray to another, from one random packing element to the next or one structured packing layer to another. For example in the case of structured packing the overall pressure drop comprises three major components: gas liquid interaction at the interface along the flow channels, flow direction change losses with associated entrance effects at the transitions between packing layers and, very influential but often ignored, the gas-gas interaction at the plane separating crossing gas flow channels. To overcome these limitations a structure that minimizes or eliminates these transitions is needed.
In addition to improvements in vapor/liquid contactors there is a further need to develop energy efficient distillation columns. One concept in saving energy is a diabatic distillation column. In this case the reboiler and condenser of a conventional column are replaced by a reboiler and condenser that are integrated within the column. This allows for the gradual addition of heat to the stripping section and removal of heat from the rectifying section. However, implementation is difficult due to large capital costs and complex tray design.
Another promising design is one that combines heat and mass transfer. The basic concept is to combine vapor compression with a diabatic distillation. Current designs involve the use of alternate passages each filled with structured packing or a shell within a shell using trays as separation devices. However, heat and mass transfer combination distillation can be significantly improved if the packing and heat exchangers are the same.
Therefore it is an object of this invention to provide an improved mass transfer capability, particularly with respect to distillation, absorption, and liquid-liquid contacting applications, based on packing with minimal surface area design.